It is known that a number of industries, e.g., the automobile industry, require parts that are both strong and lightweight. One attempt to achieve this balance between strength and minimal weight utilizes hollow parts constructed of relatively thin sheet metal. However, hollow metal parts are easily distorted. Accordingly, it is also known that the presence of structural foam in the cavities of the hollow parts can improve the strength and stiffness of such parts. For flat parts of the automotive body like doors, roofs, trunk lids or hood lids it is known to increase stiffness and rigidity of such parts by bonding sheets (“patches”) based on expandable or non expanding epoxy or polyurethane resins onto such parts.
Generally, such foams are either metallic foam materials or are prepared from formulations comprising a thermosettable resin such as an epoxy resin, a blowing agent and fillers and reinforcing agents such as hollow glass microspheres. Preferably, these foams have a density of about 0.30-0.65 g/cm3 (about 20-40 lb/ft3) and are able to withstand heat in excess of 175° C., most preferably in excess of 200° C. Optional ingredients include curatives, processing aids, stabilizers, colorants, and UV absorbers. Where automotive body structural members are reinforced with expandable thermosettable resins, the resins are frequently cured at the bake temperature of the automotive bodies during the paint coating process. However, these bake temperatures are relatively low and the structure of the automotive body can have cool spots that further inhibit the automotive body from attaining the full baking temperature which prevents some thermosettable foams from fully expanding. Additionally, the temperature within a paint bake oven tends to be rather uneven (for example, the temperature near the floor may be significantly lower than the temperature near the ceiling, meaning that thermosettable foam portions located relatively low in a vehicle body may only be exposed to a relatively low temperature). Thus, there is a need for thermosettable compositions that exhibit high degrees of expansion even at low temperatures.
EP-A-0 798 062 proposes structural components of metallic foam material where the metallic foam material is produced from a metal powder and blowing agent and is optionally shaped between massive metallic sheet metal components in a press at high temperatures and under high pressures. Such a process is suitable only for structural components of large size which are produced separately outside the assembly line of a motor vehicle and are then introduced into the normal assembly process. The incorporation and foaming of metallic foam materials is not possible under the process conditions of a normal vehicle assembly line.
U.S. Pat. No. 4,978,562 describes a specifically lightweight, reinforcing door beam of a composite material comprising a metal pipe which is partly filled by a specifically lightweight polymer with a cell structure. It is proposed to mix thermosetting resins based on epoxy resins, vinyl ester resins, unsaturated polyester resins and polyurethane resins with the corresponding curing agents, fillers and cell-forming agents in an extruder, to cure this mixture to a core and to introduce it into the metal pipe such that the core is fixed in the pipe by friction forces or mechanically. Alternatively, the polymer core can be produced by casting from liquid or paste-like polymeric material and pressed into the pipe. Reactive, thermosetting and thermally expanding shaped articles are not disclosed.
U.S. Pat. No. 4,769,391 describes a preshaped composite insert for insertion into a hollow structured body. This insert comprises a large number of thermoplastic granules of a mixture of a thermoplastic resin and non-expanded, expandable hollow microbeads and a matrix of expanded polystyrene which holds the above mentioned granules. The thermoplastic resin of the granules can be a thermoplastic here, such as, for example, a thermoplastic polyester, or it can be a thermosetting epoxy resin. After insertion of the component into the hollow body to be filled, the structural component is heated to a temperature which effects “evaporation” of the expanded polystyrene—evaporation here means breakdown of the expanded polystyrene to a thin film or carbon black. At the same time the thermoplastic granule grains expand and cure, where appropriate, hollow spaces of larger or smaller size remaining between the individual expanded granule particles, depending on the degree of expansion of the granules.
In an analogous manner, U.S. Pat. Nos. 4,861,097 and 4,901,500 describe specifically lightweight composite beams of foamed polymers and metallic structures for reinforcing vehicle doors. According to this doctrine, the polymer core component is first formed by producing a liquid or paste-like reinforcing material, which is then injected or poured into a channel-like structure and subsequently cured. Thereafter, this cured core component is introduced into the metallic hollow body structure. Alternatively, the core can be preshaped or precast by injection molding and then inserted into the hollow space.
WO 89/08678 describes a process and compositions for reinforcing structural elements, the polymeric reinforcing material being a two-component epoxy system in which the one component is a dough-like composition based on epoxy resins and the second component is a mixture of fillers, a colored pigment and a liquid curing agent of dough-like consistency. Directly before the hollow structure is filled with the reinforcing material, the two components are mixed and are introduced into the hollow body structure and cured, it being possible for the hollow body structure optionally to be preheated.
WO 96/37400 describes a W-shaped reinforcing structure which comprises a thermally expandable, resinous material and is introduced, before curing, into the hollow body to be reinforced. The reinforcing polymeric matrix preferably comprises a one-component, dough-like system comprising an epoxy resin, an acrylonitrile/butadiene rubber, fillers, high-strength glass beads, a curing agent and an accelerator and a blowing agent based on an azo compound or a hydrazide compound.
WO 98/15594 describes foamed products for uses in the automobile industry which are based on preferably liquid, two-component epoxy systems in which the one component comprises a liquid epoxy resin and metal carbonates or bicarbonates and the other component comprises pigments, optionally hollow beads and phosphoric acid. When the two components are mixed, these compositions cure with foaming. Uses for reinforcing or stiffening hollow structures are not disclosed.
The polymeric materials of the above mentioned references are either not suitable for the production of preshaped moldings which expand thermally by heating at a later point in time and are thereby thermosetting, or, if they are suitable for this, they as a rule have a very tacky surface which leads to contamination of storage areas, and on the other hand binds dust and dirt. Moreover, a tacky surface of these moldings impedes handling and in particular storage, e.g., stacking of several components on top of one another. For this reason, moldings of the references are provided with a protective film which is removed immediately before use. However, such protective films make the production and use of such moldings more expensive, and in addition the protective film must be disposed of after removal, which causes additional costs.
To reduce the surface tackiness of such moldings, WO00/52086 proposes production of thermosetting, thermally expandable shaped articles from a mixture comprising at least one solid reactive resin, at least one liquid reactive resin, at least one reactive resin having a flexibilizing action and curing agents and/or accelerators or blowing agents. These shaped articles are suitable for stiffening and/or reinforcing thin-walled metal constructions and for stiffening hollow metallic lightweight constructions. Compared with known thermosetting, thermally expandable shaped articles, the shaped articles according to the doctrine of this specification are distinguished by improved dimensional stability in the non-cured state and by a low surface tackiness. The properties of processability and dimensional stability are achieved by mixing of epoxy resins of different melting point. Nevertheless, for example, the reduced surface tackiness can still be achieved only in a temperature interval of very narrow limits, so that a formulation which is indeed non-tacky in winter has a very tacky surface in summer. Furthermore, this procedure requires the use of large amounts of expensive resins and curing systems. For inexpensive production of such expandable shaped articles by the injection molding process in particular, difficulties occur again and again in production and handling, which is undesirable for process reliability of the production process.
U.S. Pat. No. 4,444,818 thus describes a thermosetting adhesive laminated body which is built up from a thermosetting resin layer in the form of a “prepreg” and in which a reinforcing material is embedded. This specification furthermore proposes attachment to one side of the prepreg of a flattened tubular material which can resume its original tubular shape when the reinforcing laminated body is heated. The prepreg laminated body can comprise two different thermosetting resin layers. Epoxy resins are proposed as binders for the thermosetting layers of the prepreg. The tubular or hose-like body here is said to be made of polyethylene, ethylene/vinyl acetate copolymers, polypropylene, polystyrene or PVC or also nitrile rubber. The production process for such reinforcing laminated bodies is expensive.
EP-A-230666 describes a process for the production of a one-component thermosetting composition which forms a urethane-epoxy-silicone interpenetrating network (IPN) system on heating. This specification proposes production, from these compositions, of metal-reinforcing laminated bodies (“patches”) which adhere directly to oil-containing metal surfaces, such as oily steel sheets. The IPN is said to be formed here by a polyepoxy compound, a blocked polyamine curing agent and a chain-lengthened polyurethane prepolymer in which some isocyanate groups of the prepolymer are blocked with a hydroxy-functional polysiloxane.
EP-A-297036 describes a laminated body comprising a support, e.g., resin-bonded glass fiber fabric, to which a layer of thermosetting resin is applied. To protect the tacky resin surface, a cover film of a material which shrinks under the action of heat is envisaged. This film should be provided with slits which widen to open after a heat pretreatment, so that part of the tacky surface is exposed. By this means it is said to be no longer necessary to peel off the protective film before application of the laminated body. No information is given regarding the composition of the tacky resin layer.
EP-A-376880 describes a laminated body arrangement for stiffening planar bodies comprising a carrier layer of a curable synthetic resin material in which a reinforcing material bonded thereto or embedded therein is provided. An adhesive layer which comprises a curable synthetic resin material optionally provided with fillers and other additives and is applied to the carrier layer and faces the body to be stiffened is furthermore provided. To achieve the highest possible reinforcing effect without deformation of the planar body (metal sheet), the adhesive layer should have a higher elasticity modulus after curing of the synthetic resin than the cured synthetic resin material of the carrier layer, and at the same time the carrier layer and adhesive layer in the cured state should have at least approximately the same coefficient of thermal expansion as the planar body to be stiffened. The carrier layer here should comprise a glass fiber fabric and a mixture of liquid epoxy resins and solid epoxy resins and curing agents, and the adhesive layer should substantially comprise thermosetting, self-adhesive synthetic resins and is likewise built up from liquid and solid epoxy resins as well as curing agents and fillers.
EP-A-298024 similarly describes a process for stiffening metal sheets and shaped articles of plastic with the aid of a single- or multi-layered planar stiffening body in which at least one layer comprises a synthetic resin which cures under the influence of heat. This stiffening body here should initially be subjected to a first heat treatment, during which at least one surface of the stiffening body becomes tacky as a result of this first heat treatment. The stiffening body should then be applied with the tacky surface to the element to be stiffened and the stiffening body should then be subjected to a second heat treatment, until all the layers of the stiffening body have cured. It is proposed that a layer of the reinforcing body is built up from thermosetting epoxy resins and optionally comprises glass fiber fabric. An epoxide-based hot-melt adhesive, possibly based on polyurethane or copolyester, is proposed as the second layer which should become tacky during the first heat treatment. Alternatively, this layer should comprise a film which shrinks under the action of heat, so that a tacky layer is exposed after shrinkage.
WO 95/27000 describes a curable, injection-moldable composition for reinforcing thin, hard sheets of metal or plates. The composition is built up from thermosetting resins, expandable hollow microbeads and particulate reinforcing material of ground glass fibers, ground carbon fibers and mixtures thereof. The various epoxy resins based on glycidyl ethers, glycidyl esters or glycidylamines are proposed as the thermosetting resin compositions.
CA-A-2241073 describes a film-reinforcing stiffening laminate for rigid, thin-walled substrates. According to the doctrine of this specification, the polymer should cure with expansion in a lacquering oven and thereby bond intimately with the inner surface of the base substrate to be reinforced. No information regarding the binder composition is given in this specification.
As can be seen from the references described above, the sheet- or frame-reinforcing laminated bodies are substantially limited to epoxy-based systems and systems based on polyurethanes. These indeed as a rule affect the required stiffening performance, but do not meet the demand for a chemical basis which is industrially hygienic and acceptable in health terms. Reactive polyurethane systems as a general rule still comprise residues of monomeric diisocyanates. For this reason workplaces must be appropriately equipped with exhaust equipment where such compositions are used in order to be able to protect persons employed at these workplaces from exposure to isocyanates. In the case of epoxy-based systems, the dimensional stability of the components is determined by the composition of the epoxy resin mixture. The adjustment of the hardness of the laminated body in the non-cured state is determined via the nature and relative contents of the solid, semi-solid and liquid epoxy resins. The (desired) tackiness on oiled metal sheets and the resistance to washing out by the various process liquids during vehicle body production is also determined by these. High contents of low molecular weight liquid epoxy resins have hitherto been required for tacky laminated body compositions. As is known, these liquid epoxy resins comprise low molecular weight epoxide compounds with a molecular weight of less than 700. The use of such epoxy compositions is undesirable for industrial hygiene reasons, since these low molecular weight epoxide compounds can cause allergic or sensitizing reactions in contact with skin. Such non-cured laminated bodies with a high content of liquid epoxides moreover indeed have a good adhesion to the substrates to be stiffened, but they are not very resistant to the process liquids, such as washing and cleaning baths, phosphating and conversion baths and the electro-dip lacquer, and in particular the wash liquids are applied under a high pressure and at temperatures of up to 75° C. Furthermore, such laminated bodies are very flexible and therefore not very dimensionally stable and can be stacked only with expensive specific packaging.
There is thus a need for expandable resin based compositions suitable for increasing strength and stiffness for hollow parts or stiffness and rigidity for flat parts.